Poly(isobutylene-co-isoprene), or IIR, is a synthetic elastomer commonly known as butyl rubber which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (1-2 mole %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprise a major portion of isoolefin and a minor amount, not more than 2.5 mol %, of a conjugated multiolefin. Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a vehicle and a Friedel-Crafts catalyst as part of the polymerization initiator. This process is further described in U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295.
Peroxide curable butyl rubber compounds offer several advantages over conventional, sulfur-curing, systems. Typically, these compounds display extremely fast cure rates and the resulting cured articles tend to possess excellent heat resistance. In addition, peroxide-curable formulations are considered to be “clean” in that they do not contain any extractable inorganic impurities (e.g., sulfur). The clean rubber articles can therefore be used, for example, in condenser caps, biomedical devices, pharmaceutical devices (stoppers in medicine-containing vials, plungers in syringes) and possibly in seals for fuel cells.
It is well accepted that polyisobutylene and butyl rubber decompose under the action of organic peroxides. Furthermore, U.S. Pat. Nos. 3,862,265 and 4,749,505 teach us that copolymers of a C4 to C7 isomonoolefin with up to 10 wt. % isoprene or up to 20 wt. % para-alkylstyrene undergo a molecular weight decrease when subjected to high shear mixing. This effect is enhanced in the presence of free radical initiators.
In spite of this, CA 2,418,884 and 2,458,741 describe the preparation of butyl-based, peroxide-curable compounds which have high multiolefin content. Specifically, CA 2,418,884 describes the continuous preparation of IIR with isoprene levels ranging from 3 to 8 mol %. Halogenation of this high multiolefin butyl rubber produces a reactive allylic halide functionality within the elastomer. With these elevated levels of isoprene now available, it is possible, in principle, to generate BIIR analogues which contain allylic bromide functionalities ranging from 3 to 8 mol %. In essence, the relative levels of isoprene and allylic bromide can be tuned within this range. Conventional butyl rubber halogenation processes are described in, for example, Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, Volume A231 Editors Elvers, et al.) and/or “Rubber Technology” (Third Edition) by Maurice Morton, Chapter 10 (Van Nostrand Reinhold Company© 1987), particularly pp. 297-300.
In addition to enabling the co-vulcanization of halobutyl rubber with other general-purpose rubbers, the presence of allylic halide functionalities allows for nucleophilic alkylation reactions. It has been recently shown that treatment of brominated butyl rubber (BIIR) with nitrogen and/or phosphorus based nucleophiles, in the solid state, leads to the generation of IIR-based ionomers with interesting physical and chemical properties (see: Parent, J. S.; Liskova, A.; Whitney, R. A; Resendes, R. Journal of Polymer Science, Part A: Polymer Chemistry 43, 5671-5679, 2005; Parent, J. S.; Liskova, A.; Resendes, R. Polymer 45, 8091-8096, 2004; Parent, J. S.; Penciu, A.; Guillen-Castellanos, S. A.; Liskova, A.; Whitney, R. A. Macromolecules 37, 7477-7483, 2004).
Existing Butyl elastomer grades are used in a variety of applications where the inherent low gas permeation rate is of great importance. The adhesion of butyl rubber to solid surfaces is an important physical property that leads to the formation of composite materials. For example, in multi pane gas filled glass window seals, the low permeation of Butyl elastomers allows the retention of special gases of low thermally conductivity over the life of the window. As the ever-increasing demand for improved energy efficiency drives improvements in window design, better adhesion properties in window seals are required. However, existing butyl rubber polymers exhibit only moderate adhesion to glass surfaces and as a result have deficiencies when used in glass-polymer composite applications. The same is true of metal-polymer and plastic-polymer composite applications.
The publication Bayer—Manual for the Rubber industry 2nd Edition at Page 512 table D10-1 and at page 514 table D10-2 as well as page 515 table D10-4 highlights the poor adhesion of Butyl elastomers to steel, rayon, polyamide and polyester. In thermoset rubber compounds the poor adhesion of Butyl rubber is partially overcome with a laborious process of coating the fabric/steel with a resorcinol, formaldehyde, latex, isocyanate RFL bonding system. In addition a resorcinol, formaldehyde, silica RFS bonding system is incorporated into the thermoset rubber compound. Even with these efforts an adhesion rating of 3, 2, and 0 (0-5 scale, with 5 being excellent) is all that can be expected for rayon, polyamide and regular finish polyester, respectively.
There is therefore a need for improving adhesion between butyl rubber and glass, metal and/or plastic surfaces.
In the past, butyl rubber polymers have exhibited adhesion values of less than 15 psi for stainless steel, less than 10 psi for glass and less than 5 psi for mylar. Improvements in these adhesion values are constantly being sought. To date no attempts have been made to characterize adhesion between butyl rubber ionomers and glass, metal or plastic surfaces.
The need therefore still exists for a butyl polymer having improved surface adhesion characteristics and for composite articles made therefrom.